Continuous cellulose digester with charging device



Feb. 25, 1969 J. c. F. c. RICHTER 3,429,773

CONTINUOUS CELLULOSE DIGESTER WITH CHARGING DEVICE Filed April 12, 1965 l :unnnunu,

United States Patent 3,429,773 CONTINUOUS CELLULOSE DIGESTER WITH CHARGING DEVICE Johan C. F. C. Richter, St. Jean-Cap-Ferrat, France, as-

signor to Ahtieholaget Kamyr, Karlstadt, Sweden, a company of Sweden Filed Apr. 12, 1965, Ser. No. 447,179 Claims priority, application Sweden, Apr. 16, 1964, 4,682/ 64 US. Cl. 162--237 11 Claims Int. Cl. D21c 7/ 08 ABSTRACT OF THE DISCLOSURE An upright continuous cellulose digester charging device is disclosed, wherein a conduit loop connects a cellulose feeder to a casing containing a screen and a rotary screw conveyor. The casing opens into the area of the top of the digester, and the screen and the conduit loop connections in the casing are located at a level below the level of the casing opening into the area of the top of the digester. The device prevents excessive heat transfer from the digester back to the cellulose feeder.

The invention relates to an upright continuous cellulose digester with a charging device comprising a feeder operating against the digester pressure and a screening device located at a distance therefrom and consisting of a closed pressure-resistant casing, one end of which is steadily in open communication with the top of the digester and which casing encloses a rotary conveyor screw and a screen located along the latter, wherein the ends of a circulation conduit to which the feeder transfers comminuted cellulosic fiber material, such as wood chips, are connected to the casing of the screening device on opposite sides of the screen therein and in which conduit there is inserted a pump for returning to the feeder such liquid as is drawn off through said screen and for pumping a suspension of said fiber material to the screening device.

A drawback which under certain conditions may be present in previously known charging devices of said kind consists in that an excessive amount of heat is transferred from the digester to the circulating liquid and therefore to the feeder. The object of the present invention is to counteract said inconvenient heat transfer. Said object is realized essentially by the characterising feature that the two ends of the circulation conduit are connected to the casing of the screening device at a level located below the lowermost point of the fiber material outlet of the casing directly communicating with the digester top.

This involves that heat would have to be transferred in a downward direction from the digester top in order to reach the circulation conduit, which is easier to prevent than the heat transfer in the upward direction encountered earlier and caused e.g. by steam penetrating upwardly and in counter-current to the fiber material charged by the conveyor screw into the digester top, the steam heating the accompanying liquid screened off and pumped back to the feeder. A measure further obstructing such heat transfer consists in that the uppermost part of the screen is located distant from and at a lower level than the fiber material outlet of the easing into the digester. When the digester top is steam-filled, it is then possible to keep the screen completely surrounded by liquid, so that the steam cannot penetrate into the space behind the screen and heat the liquid flowing therefrom back to the feeder.

The invention will now be more closely described with reference to the accompanying drawing which diagrammatically shows a continuous digester equipped with the new charging device.

The drawing shows an upright cylindrical digester 11, the upper end of which is adapted for being charged continuously with a comminuted cellulosic material, such as wood chips, which during its motion downwardly through the digester is treated in different ways and which is discharged at the lower end of the digester through a conduit 13. Inserted in the shell of the digester is a number of screen girdles through which various liquids can be drained oif the chips column. Various conduits for supply of e.g. steam, digesting liquors, wash water etc. are also connected to the digester. The invention is not restricted to any specific combination of treatments in the digester, but in the shown embodiment there is performed in the uppermost part of the digester a prehydrolyzing digestion in steam phase which at the liquid surface level 15 changes into a prehydrolyzing digestion in liquid phase. Below the level 15 there follows a delignifying digestion by means of liquor flowing in counter-current through the middle part of the digester, and in the lowermost part of the digester there takes place a partial washing-out of the digesting liquor by means of wash water supplied to the lower end of the vessel and driven counter-currently up through the chips column. In the middle delignifying zone a temperature of the order of 170 C. should be maintained and therefore the digester is held under a correspondingly high pressure, e.g. of the order of 150 pounds per square inch.

The objects of the charging device is to charge chips etc. continuously into the upper end of the digester against the action of said high pressure.

The charging device consists of two units connected to each other by means of tube conduits, one unit consisting of a screening device located close to the digester top and in its entirety designated by the numeral 17, and the other unit consisting of a feeder designated in its entirety by 19 and usually located at a lower level than the screening device.

The feeder 19 which may be of any known type, is shown as a rotary feeder essentially consisting of a cylindrical or conical rotor 21 and a casing 23 therefor, having four connections 25, 27, 29 and 31 distributed around the circumference. The rotor is provided with a diametrically through-going pocket 33 which from the shown vertical filling position can be turned into an inclined position in which its two ends are shut off by the casing 23 and from there on into a horizontal emptying position. Attached to the upper connection 25 is a chute 35 having a perforated wall enclosed by a container 37 which is held partly filled with liquid. Through the chute there pass chips emanating from the chips bin 39 and having been fed by the charging device 411 into the steaming vessel 43 which is held under a low pressure, e.g. one atmosphere overpressure, and which chips have been steamed therein while being advanced by a motor-driven screw conveyor 45. In the lower part of the chute the chips are soaked with liquid, and in the shown position of the rotor 21 the pocket 33 is being filled with the chipsliquid-mixture. By means of a pump 47 liquid is circulated in a conduit extending between the lower connection 27 and the container 37, which aids in the filling of the pocket 33. The connection 27 is partly obstructed by a screen 49 attached to the casing 23 and serving to retain the chips in the pocket 33, whereas the liquid runs through and is returned through said circulation conduit.

When the rotor 21 is turned so that the pocket 33 is horizontal and its ends are placed opposite to the connections 29, 31 of the casing, the pocket forms apart of a circulation loop held under digester pressure and formed by two conduit branches 51, 53 and the screening device 17 at the top of the digester. By means of a pump 55 inserted in the return conduit 53 (which only carries liquid) the chips-liquid-mixture in the pocket 33 is pumped up through the conduit 51 to the screening device 17 where the chips are delivered, whereas the liquid is recirculated. On account of the pocket 33 being completely filled with liquid when it is returned to chips filling position and also on account of leakage due to the high pressure, the liquid content of the container 37 around the chute increases. However, the level therein is maintained constant by liquid being discharged into a collecting tank 57 under control of a valve 59 and a level feeling means.

The screening device 17 at the digester top comprises a pressure-resistant casing 61 which encloses a cylindrical screen 63. The casing is composed of a cylindrical part 65 having the same diameter as the screen 63 and forming an axial extension thereof, and a part 67 of a greater diameter surrounding the screen 63 in such a manner that an isolated space is formed on the outside of the screen 63. The part 67 preferably forms a spherically widened part which is joined to the lower end of the cylindrical part 65 of the casing. Arranged inside the screen 61 and the casing part 65 and concentric thereto is a motor-driven feed screw 69, the shaft 71 of which extends through the plane or bulging ends of the casing 61 and is sealed thereto.

The conduit 51 through which the chips-liquid-mixture is supplied, opens out in the casing 61 at the lower end and thereof and on the inner side of the screen 63, whereas the conduit 53 is connected to the widened part 67 surrounding the screen 63 and therefore is capable of returning to the feeder 19 merely such liquid as has drained through the screen 63. The chips are advanced by the feed screw 69 in the axial direction thereof at first along and past the screen 63 and then through the cylindrical casing part 65 on to a lateral outlet 73 positioned at or near the upper end of the casing and connected by a flange connection to the tapering top part of the digester.

According to the invention the screening device is inverted in comparison to a previously commonly used design, i.e. the feed direction of the chips therein is not downward as in the digester, but is upward, and the screening device has its communication with the digester located at a high level, whereas the chips are supplied to the screening device at a low level. Preferably the screen and the conveyor screw are inclined to the horizontal plane by an angle of between 30 and 60 degrees. Thereby, the chips outlet 73 at the upper end of the casing 61 may be placed substantially straight above the centre of the digester, as shown in the drawing, so that the chips drop down directly into the digester in the shortest possible path. It will be evident that the conduits connected to the lower end of the casing of the screening device are situated on a level which is lower than the lowermost point of the chips outlet 73, and the screen 63 is also situated in its entirety distant from and on a level lower than said lowermost point of the chips outlet 73. Preferably, the distance between said outlet 73 and said screen 63 is of the order of the axial length of the screen 63 or at least a third of the axial length of the feed screw 69.

According to the invention the screening device should preferably be so designed that the length and the inclination of its casing areso chosen relatively to the width measured at right angles to the axis of the feed screw that a free liquid surface can form at a level which is located below the level of the chips outlet of the casing but above the screen 63 and also above the two connection points of the circulation conduit 51, 53 upon the casing. The embodiment shown in the drawing possesses said feature, and as seen therein the liquid surface 75 forms a border surface which prevents the steam present thereabove and supplied to the digester top through the conduit 77, from penetrating down to the screen 63 and heating the liquid flowing in the conduit 53 back to the feeder 19. It is true that steam may-condense upon and heat the surface 75 of the liquid in the cylindrical part 65 of the screening device, but as the upper layer of said liquid is steadily renewed by adhering to the chips moving upwardly out of the liquid and as there is also no tendency to the development of convection currents in a liquid column having an upwardly rising temperature, it is evident that the liquid returned through the conduit 53 to the feeder 19 has not been subjected to any significant heating in the screening device 17.

The surface level 75 is maintained substantially constant in a position coincident with a middle part of the feed screw 69 and the cylindrical casing part 65. The device used therefor consists of a level feeding means designated by the symbol 79, which means acts upon a control valve 81 inserted in a conduit 83 through which liquid from the collecting tank 57 is returned by means of a pump 85 to the screening device 17. The quantity of liquid carried with the chips When rising above the liquid surface level 75 of the screening device is replaced by liquid from the conduit 87, which liquid is supplied to the tank 57 in such a quantity that the level therein is maintained constant. Said liquid may be either water or some treating liquid, such as digesting liquor, agreeing with the treatment to which the chips are to be subjected immediately after their introduction into the digester.

At the place where it is desired to have the free liquid surface 75, the thread of the feed screw 69 is left out, so that the chips are lifted up and out of the liquid by the action of the rising lower part of the chips column. Somewhat above the liquid surface 75 the screw feeder 69 again acts directly upon the chips in order to feed them towards the outlet 73.

The charging device shown may also be used for a digester having no gas or steam filling and the overpressure of which is maintained by hydraulic compression. Then a free liquid surface 75 is not formed and the valve 81 should be controlled otherwise, e.g. by the pressure of the digester. Thus, a digester provided with the new charging device can be operated either with or without a gas phase at the top, and conversion from one to the other kind of operation may easily take place.

Within the scope of the following claims the embodiment shown can be modified also in other respects. For instance, the steaming vessel 43 may be replaced by an upright container in which steaming as well as impregnation of the chips with digesting liquor take place. The feeder may be of the reciprocating or plunger type.

I claim:

1. An upright continuous cellulose digester charging device comprising a feeder operating against the digester pressure and a screening device located at a distance from said feeder, said screening device comprising a closed pressure-resistant casing, one end of which is in steadily open communication with the top of the digester said casing enclosing a rotary conveyor screw and a screen completely surrounding part of said screw, a circulation conduit to which said feeder transfers comminuted cellulosic fiber material, such as wood chips, being connected to said casing on opposite sides of said screen therein, a pump in said circulation conduit for returning to said feeder such liquid as is drawn off through said screen and for pumping a suspension of said fiber material to the screening device, the said circulation conduit being connected to said casing of said screening device at a level located below the lowermost point of the end of said casing directly communicating with the digester top.

2. A digester charging device as claimed in claim 1, in which the uppermost part of said screen is located distant from and lower than said fiber material outlet of said casing into the digester.

3. A digester charging device as claimed in claim 1, in which the length and the inclination of said casing are so chosen relatively to the width of the same, measured at right angles to the axis of the conveyor screw, that a free liquid surface can form at a level which is located below the level of said fiber material outlet of said casing but above said screen and also above the said two points of connection of said circulation conduit to said casing.

4. A digester charging device according to claim 3, wherein a device controls or maintains constant said liquid surface level in a position coincident with a middle part of said conveyor screw.

5. A digester charging device according to claim 3, in which the thread of said conveyor screw is left out along a comparatively small portion of the same in the area where said free liquid surface is maintained.

6. A digester charging device according to claim 1, in which the part of said casing located next to said casing end is cylindrical, in which said screen enclosing said conveyor screw and located next to said fiber material inlet is cylindrical, lies in the extension of and has the same diameter as said first-mentioned cylindrical casing part, and in which said casing has a part of greater diameter enclosing said screen.

7. A digester charging device as claimed in claim 1 wherein the distance between said fiber material outlet and said screen is at least one-third of the axial length of said screw.

8. An upright continuous cellulose digester charging device comprising a feeder, a screening device comprising a closed pressure-resistant casing, one end of which opens into the top of said digester, a screen, a rotary screw conveyor located within said casing and enclosed at its lower end by said screen, and conduit connections opening into said casing, said screen and said conduit connections located at a level below the lowermost point of said end of said casing opening into the top of said digester, and a conduit loop consisting of a feed conduit and a liquid return conduit and a pump inserted in said return conduit, said conduit loop connecting said conduit connections of said screening device to said feeder.

9. A digester charging device as claimed in claim 1, in which said screen is an enclosed cylindrical screen and said conveyor screw and said enclosed cylindrical screen are arranged with their axial direction inclined to the horizontal plane, the feed of said screw being directed obliquely upwards, and the upper end of said casing being connected directly to the top part of the digester.

10. A digester charging device as claimed in claim 9, in which said conveyor screw and said screen are inclined to the horizontal plane by an angle of to degrees.

11. A digester charging device as claimed in claim 9, in which the upper end of said casing is located substantially straight above the digester top, so that said fiber material departing from said casing drops directly down into the digester.

References Cited UNITED STATES PATENTS 2,803,540 8/1957 Durant 162246 X 2,872,314 2/1959 Sandberg 162-19 2,999,784 9/1961 Hullrnann 16219 HOWARD R. CAINE, Primary Examiner.

US. Cl. X.R. l62-246 

